Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a speaker driver including a power amplifier for driving an audio output signal to headphones or speakers. Oftentimes, a power converter may be used to provide a supply voltage to a power amplifier in order to amplify a signal driven to speakers, headphones, or other transducers. A switching power converter is a type of electronic circuit that converts a source of power from one direct current (DC) voltage level to another DC voltage level. Examples of such switching DC-DC converters include but are not limited to a boost converter, a buck converter, a buck-boost converter, an inverting buck-boost converter, and other types of switching DC-DC converters. Thus, using a power converter, a DC voltage such as that provided by a battery may be converted to another DC voltage used to power the power amplifier.
Often, boost converters operate as peak current-controlled boost converters, wherein a main control loop of a control system is used to determine a peak current requirement on each switching phase of the boost converter in order to generate a desired boosted output voltage of the boost converter. For boost duty cycles where a duty cycle (e.g., which may be determined by subtracting an arithmetic ratio from the number one, wherein the arithmetic ratio equals the input voltage supplied to the boost converter divided by the boost output voltage of the boost converter), slope compensation circuitry may be required to avoid sub-harmonic behavior of the boost converter. Also present in many boost converter control systems is protection circuitry to ensure that the current of a boost converter is maintained below a maximum value. The detection of the peak current in accordance with the main control loop and detection of the maximum allowable current is often performed by two separate circuits: a first comparator comparing a measured current (e.g., measured current of a power inductor of the boost converter) with a slope-compensated target peak current signal and a second comparator comparing the measured current to the maximum current limit. The main control loop, which may also be known as a compensator, may generate a target peak current signal which may be modified by slope compensation circuitry, and such slope-compensated target peak current signal may be compared by the first comparator to the measured current in order to perform peak-current control of a boost converter. However, because slope compensation may occur in analog circuitry, an unknown amount of correction may exist at the point the first comparator toggles. Such error may be removed by the main control loop in regulating the boosted voltage output by the power converter.
However, the presence of this unknown error may result in the inability to directly control the maximum current during any specific switching cycle of the boost converter. This is because the second comparator allows for a measurement without slope compensation of the inductor current above a threshold. If the second comparator is used to control the current in the inductor directly, the lack of slope compensation on this measurement may result in sub-harmonic behavior. To avoid such sub-harmonic behavior while limiting the current as detected by the second comparator, the output of the second comparator may be fed back to allow control circuitry to apply desired limit behavior to the slope-compensated target peak current signal. For example, an additional control loop may be present such that when operating under the current-limited condition, the slope-compensated target peak current signal is modified to obtain the desired limited current behavior.
As a result, a control system may be created that results in limiting and controlling the peak current of a power inductor of a boost converter below a maximum threshold. However, in many systems, an error between the peak inductor current and the average inductor current can be quite large and inductor variation can lead to significant challenges in determining a proper peak current limitation.